This invention relates to an improvement in plasma focus devices. In particular it relates to an improvement in the neutron yield, in the quality of the current sheath and in the consistency of the performance of a plasma focus device by empolying a novel design for a field distortion element.
Plasma focus devices are plasma coaxial accelerators designed to employ a high-current pinch effect at the end of a central conductor in order to produce high neutron fluxes. See, e.g. W. H. Bostick, V. Nardi and W. Prior, Formation and Decay of Vortex Filaments in a Plasma Current Sheath, Proc. Int. Sym. on Dynamics of Ionized Gases (1971). Plasma focus devices have utility also as pulsed particle beam accelerators, plasma accelerators, X-ray radiation sources, nuclear fusion reactors, neutron sources, and megamp opening switches.
In these devices a plasma focus is usually formed by a pair of coaxial electrodes with a sleeve of insulating material between the electrodes. The insulator sleeve closely encircles the inner electrode to within manufacturing tolerances usually of less than 1 mm and electrically separates the anode from the cathode. These electrodes are typically contained in a tank filled with suitable pressureized gas such as deuterium. The plasma focus device typically employs as an energy source a low inductance power supply such as a capacitor bank and a system of one or more low-inductance switches in the power-transmission lines between the power-supply and the electrodes, for producing megamp, microsec, electric discharges.
These electric discharges produce a shock-driving current sheath (sometimes having a corrugated filamentary structure). Deuterons and electrons present in the pinched plasma at the final stage of the discharge are accelerated at energies many times higher than the applied potential of the power supply. The current sheath advances down the length of the electrodes and in the final stage pinches-in at the end of the electrodes typically collapsing on the axial region of the discharge. Neutrons are generated in the plasma typically starting at the pinch formation up to 50 to 500 nsec after the pinch giving a relatively long neutron pulse (20-500 nsec).
A critical role in the performance of plasma focus devices is played by the quality of the plasma current sheath in the interelectrode gap of the two coaxial electrodes where the bulk of the interelectrode current is concentrated. The quality of the current sheath is described by the peak current density J.sub.m on the sheath, the reciprocal of the current sheath thickness 1/d, and the current-sheath speed of propagation v.sub.s along the electrode axis. The quality controls the efficiency of the process of concentrating and transferring the energy initially stored in the external power supply (e.g., a capacitor bank) to the plasma region at the front end (muzzle) of the electrodes, where the current sheath converges and focuses at the end of the run-down phase between the electrodes. The efficiency of the energy transfer process ordinarily increases for increasing values of J.sub.m, d.sup.-1, and v.sub.s for a given power supply capacitance and peak charging potential. In the final stage of the plasma focus discharge where the current sheath implodes on the electrode axis a plasma current channel forms in which the energy density is increased by a factor typically of 10.sup.8 as compared to the initial energy density in the capacitor bank.
The use of field distortion elements to improve the performance of plasma focus devices is known. The use of a tight fitting type of knife-edge at the breech end of a coaxial electrode for improving the plasma focus performance has been used and reported in the literature. W. H. Bostick, C. M. Luo, V. Nardi, C. Powell, Measurements on Pinhole Camera Photographs With Particle Detectors And Plasma Focus Optimization, Proc. 4th Int. Workshop On Plasma Focus and Z-Pinch Research, pp. 128-31 (Warsaw 1985); M. Borowiecki et al, Influence Of Insulator On Plasma-Focus Discharge, Ibid, pp. 86-89 (Warsaw 1985). In those applications the knife edge internal surface and the sharp edge of the cylindrical knife are resting on or are very close to (i.e. to within manufacturing tolerances less than approximately 1 mm) the outer surface of the insulator sleeve. This small-radius knife edge increases the neutron yield in deuterium by a factor between 1.3 and 2 in plasma focus systems operating with an energy from about 1 kilojoule (kJ) to about 100 kJ at optimum conditions of operation. When such knife edges are employed, however, there remains a fluctuation from shot to shot in the performance of the plasma focus device for repeated pulses (i.e. "shots") of the devices.